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.Dd $Mdocdate: April 8 2023 $
.Dt FIDO_ASSERT_SET_AUTHDATA 3
.Os
.Sh NAME
.Nm fido_assert_set_authdata ,
.Nm fido_assert_set_authdata_raw ,
.Nm fido_assert_set_clientdata ,
.Nm fido_assert_set_clientdata_hash ,
.Nm fido_assert_set_count ,
.Nm fido_assert_set_extensions ,
.Nm fido_assert_set_hmac_salt ,
.Nm fido_assert_set_hmac_secret ,
.Nm fido_assert_set_up ,
.Nm fido_assert_set_uv ,
.Nm fido_assert_set_rp ,
.Nm fido_assert_set_sig ,
.Nm fido_assert_set_winhello_appid
.Nd set parameters of a FIDO2 assertion
.Sh SYNOPSIS
.In fido.h
.Bd -literal
typedef enum {
	FIDO_OPT_OMIT = 0, /* use authenticator's default */
	FIDO_OPT_FALSE,    /* explicitly set option to false */
        FIDO_OPT_TRUE,     /* explicitly set option to true */
} fido_opt_t;
.Ed
.Ft int
.Fn fido_assert_set_authdata "fido_assert_t *assert" "size_t idx" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_authdata_raw "fido_assert_t *assert" "size_t idx" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_clientdata "fido_assert_t *assert" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_clientdata_hash "fido_assert_t *assert" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_count "fido_assert_t *assert" "size_t n"
.Ft int
.Fn fido_assert_set_extensions "fido_assert_t *assert" "int flags"
.Ft int
.Fn fido_assert_set_hmac_salt "fido_assert_t *assert" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_hmac_secret "fido_assert_t *assert" "size_t idx" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_up "fido_assert_t *assert" "fido_opt_t up"
.Ft int
.Fn fido_assert_set_uv "fido_assert_t *assert" "fido_opt_t uv"
.Ft int
.Fn fido_assert_set_rp "fido_assert_t *assert" "const char *id"
.Ft int
.Fn fido_assert_set_sig "fido_assert_t *assert" "size_t idx" "const unsigned char *ptr" "size_t len"
.Ft int
.Fn fido_assert_set_winhello_appid "fido_assert_t *assert" "const char *id"
.Sh DESCRIPTION
The
.Nm
set of functions define the various parameters of a FIDO2
assertion, allowing a
.Fa fido_assert_t
type to be prepared for a subsequent call to
.Xr fido_dev_get_assert 3
or
.Xr fido_assert_verify 3 .
For the complete specification of a FIDO2 assertion and the format
of its constituent parts, please refer to the Web Authentication
(webauthn) standard.
.Pp
The
.Fn fido_assert_set_count
function sets the number of assertion statements in
.Fa assert
to
.Fa n .
.Pp
The
.Fn fido_assert_set_authdata
and
.Fn fido_assert_set_sig
functions set the authenticator data and signature parts of the
statement with index
.Fa idx
of
.Fa assert
to
.Fa ptr ,
where
.Fa ptr
points to
.Fa len
bytes.
A copy of
.Fa ptr
is made, and no references to the passed pointer are kept.
Please note that the first assertion statement of
.Fa assert
has an
.Fa idx
of
.Em 0 .
The authenticator data passed to
.Fn fido_assert_set_authdata
must be a CBOR-encoded byte string, as obtained from
.Fn fido_assert_authdata_ptr .
Alternatively, a raw binary blob may be passed to
.Fn fido_assert_set_authdata_raw .
.Pp
The
.Fn fido_assert_set_clientdata_hash
function sets the client data hash of
.Fa assert
to
.Fa ptr ,
where
.Fa ptr
points to
.Fa len
bytes.
A copy of
.Fa ptr
is made, and no references to the passed pointer are kept.
.Pp
The
.Fn fido_assert_set_clientdata
function allows an application to set the client data hash of
.Fa assert
by specifying the assertion's unhashed client data.
This is required by Windows Hello, which calculates the client data
hash internally.
For compatibility with Windows Hello, applications should use
.Fn fido_assert_set_clientdata
instead of
.Fn fido_assert_set_clientdata_hash .
.Pp
The
.Fn fido_assert_set_rp
function sets the relying party
.Fa id
of
.Fa assert ,
where
.Fa id
is a NUL-terminated UTF-8 string.
The content of
.Fa id
is copied, and no references to the passed pointer are kept.
.Pp
The
.Fn fido_assert_set_extensions
function sets the extensions of
.Fa assert
to the bitmask
.Fa flags .
At the moment, only the
.Dv FIDO_EXT_CRED_BLOB ,
.Dv FIDO_EXT_HMAC_SECRET ,
and
.Dv FIDO_EXT_LARGEBLOB_KEY
extensions are supported.
If
.Fa flags
is zero, the extensions of
.Fa assert
are cleared.
.Pp
The
.Fn fido_assert_set_hmac_salt
and
.Fn fido_assert_set_hmac_secret
functions set the hmac-salt and hmac-secret parts of
.Fa assert
to
.Fa ptr ,
where
.Fa ptr
points to
.Fa len
bytes.
A copy of
.Fa ptr
is made, and no references to the passed pointer are kept.
The HMAC Secret
.Pq hmac-secret
Extension is a CTAP 2.0 extension.
Note that the resulting hmac-secret varies according to whether
user verification was performed by the authenticator.
The
.Fn fido_assert_set_hmac_secret
function is normally only useful when writing tests.
.Pp
The
.Fn fido_assert_set_up
and
.Fn fido_assert_set_uv
functions set the
.Fa up
(user presence) and
.Fa uv
(user verification)
attributes of
.Fa assert .
Both are
.Dv FIDO_OPT_OMIT
by default, allowing the authenticator to use its default settings.
.Pp
The
.Fn fido_assert_set_winhello_appid
function sets the U2F application
.Fa id
.Pq Dq U2F AppID
of
.Fa assert ,
where
.Fa id
is a NUL-terminated UTF-8 string.
The content of
.Fa id
is copied, and no references to the passed pointer are kept.
The
.Fn fido_assert_set_winhello_appid
function is a no-op unless
.Fa assert
is passed to
.Xr fido_dev_get_assert 3
with a device
.Fa dev
on which
.Xr fido_dev_is_winhello 3
holds true.
In this case,
.Em libfido2
will instruct Windows Hello to try the assertion twice,
first with the
.Fa id
passed to
.Fn fido_assert_set_rp ,
and a second time with the
.Fa id
passed to
.Fn fido_assert_set_winhello_appid .
If the second assertion succeeds,
.Xr fido_assert_rp_id 3
will point to the U2F AppID once
.Xr fido_dev_get_assert 3
completes.
This mechanism exists in Windows Hello to ensure U2F backwards
compatibility without the application inadvertently prompting the
user twice.
Note that
.Fn fido_assert_set_winhello_appid
is not needed on platforms offering CTAP primitives, since the
authenticator can be silently probed for the existence of U2F
credentials.
.Pp
Use of the
.Nm
set of functions may happen in two distinct situations:
when asking a FIDO2 device to produce a series of assertion
statements, prior to
.Xr fido_dev_get_assert 3
(i.e, in the context of a FIDO2 client), or when verifying assertion
statements using
.Xr fido_assert_verify 3
(i.e, in the context of a FIDO2 server).
.Pp
For a complete description of the generation of a FIDO2 assertion
and its verification, please refer to the FIDO2 specification.
An example of how to use the
.Nm
set of functions can be found in the
.Pa examples/assert.c
file shipped with
.Em libfido2 .
.Sh RETURN VALUES
The
.Nm
functions return
.Dv FIDO_OK
on success.
The error codes returned by the
.Nm
set of functions are defined in
.In fido/err.h .
.Sh SEE ALSO
.Xr fido_assert_allow_cred 3 ,
.Xr fido_assert_verify 3 ,
.Xr fido_dev_get_assert 3 ,
.Xr fido_dev_is_winhello 3
